In a magnetic data storage and retrieval system, a magnetic recording head typically includes a read head having a magnetoresistive (MR) sensor for retrieving magnetically encoded information stored on a magnetic disc. Magnetic flux from the surface of the disc causes rotation of the magnetization vector of a sensing layer of the MR sensor, which in turn causes a change in electrical resistivity of the MR sensor. The change in resistivity of the MR sensor can be detected by passing a bias current through the MR sensor and measuring a voltage across the MR sensor. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary to recover the information encoded on the disc.
The essential structure in contemporary read heads is a thin film multilayer containing ferromagnetic material that exhibits some type of magnetoresistance. Examples of magnetoresistive phenomena include anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunneling magnetoresistance (TMR). A typical GMR read sensor configuration is the GMR spin valve, in which the GMR read sensor is a multi-layered structure formed of a nonmagnetic spacer layer positioned between a ferromagnetic pinned layer and a ferromagnetic free layer. The magnetization of the pinned layer is fixed in a predetermined direction, typically normal to an air bearing surface of the GMR read sensor, while the magnetization of the free layer rotates freely in response to an external magnetic field. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the pinned layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect (i.e., greater sensitivity and higher total change in resistance) than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer.
When the recording head is scanned over a disc, the free layer magnetization will rotate in response to the stray fields emerging from the bits in the media, producing changes in resistance. However, thermal energy induces stochastic fluctuations in the free layer magnetization that results in random resistance fluctuations. This phenomenon is referred to as mag-noise (see N. Smith and P. Arnett, Appl. Phys. Lett. 78, 1448 (2001)) and it originates from the thermal excitation of spin waves (the variation of the magnetization in a ferromagnetic material due to precession of the spins that make up the magnetization) in the free layer. Although readers operate in a frequency bandwidth that lies below the resonant frequency of the spin wave modes, the intrinsic damping of these modes leads to significant fluctuation amplitude below resonance. A theoretical description of this phenomenon (see V. L. Safonov and H. N. Bertram, Phys. Rev. B Vol. 65, 172417 (2002)) expresses the spectral noise power as
                                          S            V                    =                                    C              V              2                        ⁢                                          γ                ⁢                                                                  ⁢                                  k                  B                                ⁢                T                                                              M                  S                                ⁢                V                                      ⁢                                          (                                                                            H                      K                                        +                                          4                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      M                      ⁢                                                                                          ⁢                      s                                        +                    H                                                                              H                      K                                        +                    H                                                  )                                      ⁢                                          α                ⁢                                                                  ⁢                2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                γ                ⁢                                                                  ⁢                Ms                                            ω                0                                      ⁢                          F              ⁡                              (                ω                )                                                    ,                            (                  Eq          .                                          ⁢          1                )            where the coefficient Cv is given by
                                          C            V                    =                                    I              b                        ⁢                                          ∂                R                                            ∂                H                                      ⁢                                          ∂                H                                            ∂                m                                                    ,                                  ⁢        and                            (                  Eq          .                                          ⁢          2                )                                          F          ⁡                      (            ω            )                          =                              1                                                            (                                                            ω                      0                                        -                    ω                                    )                                2                            +                                                (                                      α                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    γ                    ⁢                                                                                  ⁢                                          M                      S                                                        )                                2                                              +                                    1                                                                    (                                                                  ω                        0                                            +                      ω                                        )                                    2                                +                                                      (                                          α                      ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      γ                      ⁢                                                                                          ⁢                      Ms                                        )                                    2                                                      .                                              (                  Eq          .                                          ⁢          3                )            In Equations 1, 2, and 3, γ is the gyromatic ratio; kB is Boltzmann's constant; T is temperature; MS, V, and HK, are respectively the saturation magnetization, volume, and uniaxial anisotropy of the free layer; H is the external magnetic field; α is the Gilbert damping coefficient from the Landau-Lifshitz equation (a dimensionless coefficient typically having a range of 0.004 to 0.15 for physical materials); Ib is the bias current; R is the device resistance; and m is the normalized free layer magnetization. This formulation assumes excitation of only the lowest order, uniform spin wave mode having a resonant frequency of ω0/2π. Additional assumptions that factor into Equation 1 are that the free layer is biased orthogonally to the pinned layer and that the angular fluctuations of the free layer are small.
The primary implication of this phenomenon is that the noise amplitude scales linearly with the bias current and can become significantly larger than the Johnson noise (the noise generated by thermal agitation of electrons in a conductor). This creates a problem because any potential degradation of the reader signal-to-noise ratio can negatively impact the performance of the overall recording system. Further compounding this problem is that, as Equation 1 implies, the noise will increase as the reader volume decreases. Thus, without significant modifications in design, the noise will increase with continued increasing areal density.
The solution to this problem will therefore entail a method to reduce the mag-noise within the reader bandwidth. One approach to reduce the noise is to increase the anisotropy field of the free layer so that the resonant frequency is shifted to higher frequencies. However, this stiffens the free layer magnetization such that reader sensitivity is adversely affected, and the signal-to-noise ratio will likely be little changed from its initial state. The best procedure is to implement an approach that will reduce the noise without degrading the reader amplitude.
For frequencies below resonance (ω>>ω0), the noise amplitude is directly proportional to the damping coefficient, α (see Equation 1). Reader noise will therefore decrease as α is reduced. It is possible to tailor the material properties of the free layer in order to engineer a reduction in α, but this is a very difficult approach that is potentially hard to control. The present invention reduces reader noise by means of a simpler and more controllable approach.